Monday, November 01, 2010

What's the best way to pack rice, coffee beans, or other granular materials? It's an important question for companies selling grain products or people in a rush to buy coffee, nuts or raisins who want to pack as much into a bag as quickly as they can.

In the past, scientists typically only worried about two things when packing granular materials in a container: The number of grains and the volume of the container. However, new theories arose saying the amount of stress in the system ought to also be considered.

Imagine you have a large can half full of coffee beans and you tap it very gently. After each tap, you will see the beans bounce very little before settling back into place. In this case, the beans fall from a short height and the stress they place on one another (or how much they press against each other) after the fall is said to be small. If, however, you give the can a good thump, the beans will bounce much higher and fall much farther allowing you to pack them tightly in less time.

To test the theory, a group of scientists from Argentina, Spain and Venezuela built a mock 2D container about an inch long and almost six inches tall and filled it with one thousand beads, each about the diameter of a grain of rice. The container was just wider than the diameter of one bead, allowing the beads enough wiggle room to slip from the sides of the container.

The team used an mechanical shaker to tap the container at different intensities. At a high intensity, the beads settled into place after just a few taps. However, when the intensity was reduced to about a fifth of the original strength, it took almost 100 taps to reach the same packing.

The experiment was repeated in a 3D container and had the same results. A few strong taps could settle granular material as efficiently as many small taps. The same behavior can be seen at home with a plastic container of trail mix. After a few hefty taps, the nuts and raisins will pack together tightly. Loosen them up and give them a hundred tiny taps and they'll pack together again, but at the expense of time.

Accordingly, this new data might help companies packing granular materials pack just as effectively as before but in less time. A formula considering stress as well as the number of granules and the volume of the container could also help someone interested in packing less material in a give volume, like a filter manufacturer, since the formula can give the optimal tapping strength for a particular density.

Though this discovery is something you might have stumbled upon in your own kitchen, it did come as a surprise to some in the granular physics community where little experimental research has been done. The next time you're alone with a can of coffee or a tub full of trail mix, give it a few taps to see it for yourself.